Bring a large pot of salted water to a boil. Add potatoes and cook until tender but still firm, about 20-30 minutes. Drain, cool, peel, and chop. Combine the potatoes, chopped eggs, celery, and onion. Whisk together the mayonnaise, mustard, salt, and pepper. Add dressing to potato mixture, toss well to coat. Refrigerate and serve chilled.